Scale downs can be triggered by configured scale down policies, scheduled scaling, or user action. Whenever a scale down occurs, the instance is terminated according to the configured termination policy.

A scale down as a result of a scale-down policy causes a message similar to the following in the scaling history:

At 2016-05-08T13:55:14Z a monitor alarm My-Scale-Down-Alarm in state ALARM triggered policy Decrease Group Size changing the desired capacity from 4 to 3.

You can adjust your Auto Scaling group scaling policy using the AWS console or with the AWS CLI put-scaling-policy command. To configure when a scale down occurs, adjust the associated CloudWatch alarm or create a new alarm and associate the new alarm with the Auto Scaling group scaling policy.